Deflection system for cathode-ray tubes

ABSTRACT

An electromagnet deflection system for CRT&#39;&#39;s has an energystoring inductor connected between a voltage supply and a center tapping of a double-ended deflection coil. The system includes circuits responsive to the potentials at the ends of the coil, and when a rapid beam deflection occurs, the passage of one or other potential beyond a predetermined level is detected by the circuits which then cause energy to be removed from the inductors. Energy is replenished in the inductor either when a deflection is over, or in a modification having a switch regulator, simultaneously with the withdrawal of energy from the inductor.

United States Patent 1111 3,532,70

[72] Inventor David Hartley Thomas [56] References Cited N England UNITED STATES PATENTS [21] P 3,155,873 11/1964 Paschal 315/27 [22] 1969 3 426 241 2/1969 P k 315/27 Patemed June 1,1971 er ms [73] Assignee A.C. Cossor Limited Primary ExaminerRichard A. Farley Harlow Esex, England Assistant Examiner-J. M. Potenza [54] DEFLECTION SYSTEM FOR CATHODE-RAY Attorney-Roberts, Cushman & Grover ABSTRACT: An electromagnet deflection system for CRTs has an energy-storing inductor connected between a voltage supply and a center tapping of a double-ended deflection coil. The system includes circuits responsive to the potentials at the ends of the coil, and when a rapid beam deflection occurs, the passage of one or other potential beyond a predetermined level is detected by the circuits which then cause energy to be removed from the inductors. Energy is replenished in the inductor either when a deflection is over, or in a modification having a switch regulator, simultaneously with the withdrawal of energy from the inductor.

DEF LECTION SYSTEM FOR CATHODE-RAY TUBES The present invention relates to a deflection system for the electromagnetic coils of a cathode-ray tube. The invention is particularly useful in relation to radar displays and oscilloscopes in which the beam may be required intermittently to execute a rapid deflection from a datum position. Although CRT deflection systems in radar displays andv oscilloscopes are normally electromagnetic, electromagnetic deflection gives rise to a problem in that it requires the stored energy in the deflection coils to be changed and a rapid deflection requires a rapid energy change. This in turn requires a high supply voltage since the speed is governed by the relation dI a where dI/dt is the rate of change of current in the deflection coil of inductance L and V is the supply voltage.

The disadvantage of employing a high supply voltage is that the power dissipation of the circuit becomes large, the high voltage being continuously applied.

The object of this invention is to provide a deflection system which enables a lower supply voltage to be used, for a given speed of deflection, than in known systems.

According to the present invention a deflection system for a cathode-ray tube comprises a deflection coil the wholeor part of which is connected at one end through a current amplifier to one of two terminals for connection to a source of operating potential and at the other end through an inductor to the other of the two terminals, the amplifier being responsive to an input signal to vary the current in the coil, a further current amplifier connected between the said one terminal and the said other end of the coil or part thereof, and a circuit arranged so to control the further amplifier in response to the potential on the said one end of the coil or part thereof that, when the current through the first said amplifier rises sufficiently to cause the potential at the said one end to pass beyond a predetermined level, the further amplifier is caused to conduct less strongly.

The effect of this arrangement is that, when a heavy current is required in the deflection coil and the further amplifier conducts less strongly, energy is removed from the inductor. Accordingly the operating potential can be lower than would otherwise be required, thus reducing the power dissipation in the first said amplifier. The amplifiers are preferably junction transistors.

Preferably a center-tapped deflection coil is used with two first current amplifiers connected to its two ends respectively and arranged to operate in push-pull in response to the input signal, the inductor and the further current amplifier being connected to the center tap of the coil and the said control circuit being arranged to cause the further amplifier to conduct less strongly when the potential at either end of the coil passes beyond the predetermined level. A feature of the system as described this far is that the required magnitude of the operating potential is dependent on the number of fast deflections required per second, and the inductor must be rather large to cope with a group of several large deflections before its energy can be replenished. A further improvement in the invention enables the energy to be restored continually to the inductor as energy is withdrawn from it.

In this further improvement the inductor also serves as the series-smoothing inductor of a current regulator circuit. In this improved deflector system when electrical energy is transferred from the inductor to the deflection coilduring a rapid deflection, the current regulator circuit, which preferably util-' izes a junction transistor as a series-switching element, causes current to be withdrawn from the operating potential source in order to replace the energy stored in the inductor. In-this way a still lower operating potential is satisfactory and therefore the power dissipated is further reduced, since energy is replaced while it is being withdrawn, so that the minimum permissible time between fast deflections is reduced.

The present invention will now be described in more detail, by way of example, with reference to the accompanying drawings in which:

FIG. 1 is a circuit diagram of a push-pull deflection system known in the art,

FIG. 2 is a circuit diagram of a system embodying the invennon,

FIG. 3 is a circuit diagram of a further system embodying the invention and an improvement thereto, and

FIG. 4 is a sketch of some waveforms corresponding to the circuit in FIG. 2.

A known push-pull arrangement will firstly be described and then it will be shown how the invention is applicable to snch an arrangement, which has the advantage that a single power supply only is needed for the coil. It will be appreciated however that the invention is also applicable to a single-ended arrangement.

The known arrangement of FIG. 1 has a deflection coil 10 with a center tap 11 connected to the high-voltage terminal 16 of two terminals 16 and 1 7 between which the operating potential V, is applied. The two ends of the coil are connected through two transistors 12 and 13 respectively to the low-voltage terminal 17. The current in the coil is controlled by a signal applied to a terminal 14 driving the bases of the transistors in push-pull by way of a phase splitter circuit 15. High supply voltages are required for high-speed operation, as mentioned earlier, which is disadvantageous and moreover the coil current tends to overshoot and may take many microseconds to settle, which time may be a significant proportion of the total deflection time.

A first embodiment of the invention is shown in FIG. 2. The deflection coil 10 is driven by the transistors 12 and 13 which are run in Class A, their bases being fed as in FIG. 1. The transistors carry a total current 1,. A shunt transistor 22 is connected between the center tap ll of the coil 10 and the lowvoltage terminal 17 and an inductor 21 is connected between the center tap l1 and the terminal 16. The transistor 22 normally draws a current 1 which is preferably approximately 1 /3. The inductor 21 carries the total current I =I,+I.

Under steady state conditions the potential V applied to the center tap 11 is the potential V, across the terminals 16 and 17, V, being less than V in FIG. 1.

However when the input signal changes, the currents in the transistors 12 and 13 change differentially and the potentials of their collectors differ from V,. If the collector current in the transistor 12 increases, then its collector potential will fall below V,. An auxiliary control circuit 23 is provided which reduces the current 1 flowing through the transistor 22 when-' ever the collector potential in the transistor 12 falls below a threshold value V,,,,,,. This decrease in 1 causes a decrease in I,,, so that the potential V applied to the deflector coil rises; Some of the energy for the deflection is thus removed from the inductor 21.

The circuit 23 is of the nature of a threshold or'clipper circuit which only passes a signal to the base of the transistor 22 when the collector potential of the transistor 12 falls below V,,,,,,. A like circuit 24 is provided for the collector of transistor 13 so that the current in the transistor 22 is reduced irrespective of which collector potential falls. Alternatively a single threshold circuit could'be used'with an OR input arrangement making it responsive to the lower of the two collector potentials.

The operation may be clarified'by reference to FIG; 4 in which A is an input'waveform comprising a slow ramp 40 and a much faster ramp 41*. Waveform'B is the voltage waveform for the collector of transistor 12, and waveform'C is the corresponding waveform for the collector potential of transistor 13'. WaveformC is inverted with respect to B since the two transistors operate in push-pull. A slow ramp requires a very small voltage across the coil to produce the necessary rate .of change of current, so that a high supply voltage is not needed. However a fast ramp waveform requires a much higher coil voltage for a high "rate of change of current, which causes the collector potential of'one transistor to fall below V The shunt transistor 22 is therefore partially turned'off' until the fast ramp 41 is complete, after which it turns on to reduce the value of voltage V to a value less than V,, allowing energy to be replenished in the inductor. The inductor output voltage waveform is shown in wavefon'n D.

This amplifier has the advantage that it operates on a low voltage V,, but high voltage is available as required. However the inductor must have a high inductance since several fast deflections may be required before the energy used can be replenished, and the supply voltage required depends on the number of fast deflections required per second.

An improvement on this system is shown in FIG. 3. The inductor 21 provides the large voltages as required and also acts as the smoothing inductor of a switching regulator arranged to maintain the energy stored in the inductor by drawing current from the supply only as required. When energy is withdrawn from the inductor, 1;, falls and this results in a proportional drop in potential across a resistor 31 in series with the inductor. This is provided with a conventional control circuit 34 sensitive to changes in potential by which means a switching transistor 32 also in series with the inductor is controlled. When 1;, falls to [lg-1.1]. (61;, 1 a signal from the control circuit switches this transistor on allowing current to be drawn from the supply to raise the inductor current to [I +1 whereon the transistor switches otf. A diode 33 supplies the output current until 1 falls below the switching current [1 1 when the transistor 32 switches on again.

Hence the current I is kept to the correct level at all times and the power taken from the supply is only that actually required. Desirable features of this system are that the supply voltage need not be stabilized, high speeds being obtained from lower supply voltages than are commonly used, and the circuit is furthermore largely proof against short circuits. In the area enclosed in dash lines in FIG. 3, short circuits to earth are harmless as the switching regulator keeps the input current constant, thus facilitating servicing.

I claim:

1. A deflection system for a cathode-ray tube comprising:

a deflection coil connected across a pair of voltage supply terminals,

a current amplifier between one end of said coil and one of the supply terminals to vary the current in said coil,

means for applying a controlling input signal to said current amplifier whereby the current in said coil is varied,

an energy-storing inductor connected in series with said coil and the other of the supply terminals,

a further current amplifier connected to said one of supply terminal and to said coil remote from the connection with the first said current amplifier, and

a control circuit responsive to the potential at the said one end of said coil to control the conduction of said further current amplifier,

whereby an increase in current through the first said current amplifier resulting in the potential at the said one end of said coil passing beyond a predetermined level is detected by said circuit which then causes said further current amplifier to conduct less strongly.

2. A deflection system according to claim 1, wherein said coil has a center tap which is connected through the inductor to said other supply terminal.

3. A deflection system according to claim 2, wherein there are two current amplifiers connected respectively between the two opposite ends of said coil and the said one of the supply terminals.

4. A deflection system according to claim 3, wherein separate control circuits, individual to each of said two current amplifiers, are connected to said further amplifier to control the same in response to the potentials at the two opposite ends of said coil.

5. A deflection system according to claim 3, wherein a single control circuit is connected through an OR" gate to said further amplifier to control the same, whereby both the potentials at the two opposite ends of said coil are applied to the single control circuit.

6. A deflection system according to claim 1, further includ ing a switchin re ulator circuit connected between said inductor and sai ot er supply terminal, whereby the switching regulator circuit passes current from the voltage supply to said inductor, when energy is withdrawn therefrom.

7. A deflection system according to claim 6, wherein said switching regulator circuit comprises a resistor, a switching transistor, and a switching control circuit responsive to changes in potential drop across said resistor, said switching transistor being connected between said resistor and said other supply terminal.

8. A deflection system according to claim 7, wherein a shunt diode is provided to supply current to said inductor until said switching transistor switches on, said shunt diode being connected between the said one supply terminal and the supply side of said resistor.

9. A deflection system according to claim 1, wherein the current amplifiers are junction transistors.

10. A deflection system for a cathode-ray tube comprising:

a deflection coil connected from a center tapping thereof to a first of two supply terminals,

two junction transistor current amplifiers connected between the ends of said coil and a second of said two supply terminals,

means for applying a controlling input signal to said two junction transistors whereby the current in said coil is varied,

an energy-storing inductor connected in series with said center tapping and said first supply terminal,

a further junction transistor current amplifier connected between said second supply terminal and said center tapping,

a control circuit responsive to the collector potentials of said two junction transistors which is connected to the base of said further junction transistor to reduce the conduction thereof when either of said collector potentials crosses a threshold value,

whereby an increase in current through either of said two junction transistors resulting in the potential at the respective end of said coil passing beyond said threshold value is detected by said control circuit which then causes said further current amplifier to conduct less strongly.

11. A deflection system according to claim 10, further ineluding a switching regulator circuit connected between said inductor and said other supply terminal, whereby the switching regulator circuit passes current from the voltage supply to said inductor, when energy is withdrawn therefrom.

12. A deflection system according to claim 11, wherein said switching regulator circuit comprises a resistor, a switching transistor, a switching control circuit responsive to changes in potential drop across said resistor, said switching transistor being connected between said resistor and said first supply terminal, and a shunt diode connected between said second supply terminal and the supply side of said resistor whereby said diode supplies current to said inductor until said switching transistor switches on. 

1. A deflection system For a cathode-ray tube comprising: a deflection coil connected across a pair of voltage supply terminals, a current amplifier between one end of said coil and one of the supply terminals to vary the current in said coil, means for applying a controlling input signal to said current amplifier whereby the current in said coil is varied, an energy-storing inductor connected in series with said coil and the other of the supply terminals, a further current amplifier connected to said one of supply terminal and to said coil remote from the connection with the first said current amplifier, and a control circuit responsive to the potential at the said one end of said coil to control the conduction of said further current amplifier, whereby an increase in current through the first said current amplifier resulting in the potential at the said one end of said coil passing beyond a predetermined level is detected by said circuit which then causes said further current amplifier to conduct less strongly.
 2. A deflection system according to claim 1, wherein said coil has a center tap which is connected through the inductor to said other supply terminal.
 3. A deflection system according to claim 2, wherein there are two current amplifiers connected respectively between the two opposite ends of said coil and the said one of the supply terminals.
 4. A deflection system according to claim 3, wherein separate control circuits, individual to each of said two current amplifiers, are connected to said further amplifier to control the same in response to the potentials at the two opposite ends of said coil.
 5. A deflection system according to claim 3, wherein a single control circuit is connected through an ''''OR'''' gate to said further amplifier to control the same, whereby both the potentials at the two opposite ends of said coil are applied to the single control circuit.
 6. A deflection system according to claim 1, further including a switching regulator circuit connected between said inductor and said other supply terminal, whereby the switching regulator circuit passes current from the voltage supply to said inductor, when energy is withdrawn therefrom.
 7. A deflection system according to claim 6, wherein said switching regulator circuit comprises a resistor, a switching transistor, and a switching control circuit responsive to changes in potential drop across said resistor, said switching transistor being connected between said resistor and said other supply terminal.
 8. A deflection system according to claim 7, wherein a shunt diode is provided to supply current to said inductor until said switching transistor switches on, said shunt diode being connected between the said one supply terminal and the supply side of said resistor.
 9. A deflection system according to claim 1, wherein the current amplifiers are junction transistors.
 10. A deflection system for a cathode-ray tube comprising: a deflection coil connected from a center tapping thereof to a first of two supply terminals, two junction transistor current amplifiers connected between the ends of said coil and a second of said two supply terminals, means for applying a controlling input signal to said two junction transistors whereby the current in said coil is varied, an energy-storing inductor connected in series with said center tapping and said first supply terminal, a further junction transistor current amplifier connected between said second supply terminal and said center tapping, a control circuit responsive to the collector potentials of said two junction transistors which is connected to the base of said further junction transistor to reduce the conduction thereof when either of said collector potentials crosses a threshold value, whereby an increase in current through either of said two junction transistors resulting in the potential at the respective end of said coil passing beyond said threshold value is detected by said control ciRcuit which then causes said further current amplifier to conduct less strongly.
 11. A deflection system according to claim 10, further including a switching regulator circuit connected between said inductor and said other supply terminal, whereby the switching regulator circuit passes current from the voltage supply to said inductor, when energy is withdrawn therefrom.
 12. A deflection system according to claim 11, wherein said switching regulator circuit comprises a resistor, a switching transistor, a switching control circuit responsive to changes in potential drop across said resistor, said switching transistor being connected between said resistor and said first supply terminal, and a shunt diode connected between said second supply terminal and the supply side of said resistor whereby said diode supplies current to said inductor until said switching transistor switches on. 